The present invention relates to liquid crystal cells and more particularly those using a smectic phase liquid crystal for display or image reproduction purposes.
Liquid crystal screens are systems modulating the incident light on the screen. It may be a question of flat screens with direct vision or matrix screens used with a strioscopic system for the projection of recorded images.
Liquid crystals are formed of long molecules able to be orientated under the effect of an electric field with greater or lesser ease depending on the conditions of application of the field. Liquid crystals are known which, at a high temperature, have an isotropic phase and, at a lower temperature, a succession of nematic and smectic phases.
In the case of smectic liquid crystal devices using a mixed thermal and electric effect, writing is achieved by applying an electric field during cooling of the liquid crystal from the isotropic phase to the smectic phase and during its passage into the nematic phase. A typical example of material used for this effect is cyano-octyl-4-4'-biphenyl of formula: ##STR1## which presents the following phase transitions: ##STR2##
It will be noted that this material has a nematic phase extending over 8.degree. C. If a display screen with such a material is maintained a little below the smectic-nematic transition temperature, it will be necessary to supply it with the heating power required for raising its temperature by about 10.degree. or so. For reasons of control power gain, it is therefore recommended to reduce the nematic phase as much as possible and even to suppress it. Liquid crystals are known which are mixtures of several smectic materials, presenting up to a certain percentage of one of their components a nematic phase but which do not present such a phase beyond this percentage. Such materials may be used in laser beam display devices without application of an electric field. In fact, to remain within the electric field values able to be withstood by the liquid crystal layer, it is important for the field to be applied for a sufficiently long time for it to act efficiently, which is not possible with the materials known up to present and which do not have a nematic phase. The need to provide, in mixed thermal and electric effect devices, smectic liquid crystals having a nematic phase during the smectic-isotropic transition involves considerable heating power.
To overcome these drawbacks, use has been proposed of a smectic liquid crystal which, particularly by addition of specific dopes, no longer has a nematic phase but which can be written in nevertheless by a mixed thermal and electric effect. Contrary to what was believed, the suppression of the nematic phase does not have serious consequences. The introduction of such dopes into a smectic material modifies the operating characteristics and acts on the time during which the electric field may be applied.